1. Field of the Invention
The present invention relates to a pattern inspection apparatus for detecting a pattern defect in a reticle or a mask used for the manufacture of a semiconductor integrated circuit or a liquid crystal display apparatus and, more particularly, to a pattern inspection apparatus having a function of rounding (definition: rounding corners) reference pattern data.
2. Description of the Related Art
In the manufacture of a semiconductor integrated circuit, if a defect such as a pattern disconnection exists in a photomask used for pattern transfer, a desired semiconductor element cannot be obtained, resulting in a reduction in yield. A pattern inspection apparatus, therefore, has been used to inspect a pattern defect or the like in a photomask formed by an electron beam drawing unit and so on. This apparatus is designed to detect an optical signal corresponding to a pattern formed on a photomask by radiating light on the mask and compare/collate the detected signal with a reference signal obtained from design data used to form the pattern on the mask, thereby inspecting the presence/absence of a pattern defect in the mask and the validity of the pattern.
FIG. 33 is a block diagram showing a schematic arrangement of a conventional pattern inspection apparatus. In this apparatus, a table 2 on which a photomask 1 is placed is continuously moved in the X or Y direction to perform inspecting in units of stripes 3. The table 2 is then moved in a direction perpendicular to the continuously moving direction by an amount corresponding to the stripe width, and repeatedly performs inspecting in units of stripes, thereby inspecting the entire pattern formation region on the photomask 1.
In this stripe inspecting testing, an optical signal corresponding to the pattern formed on the photomask 1 is detected by an optical sensor 4 to obtain observation value A, while design data B used to form the pattern on the photomask 1 is read out from a computer 5 to prepare bit pattern data C' shown in FIG. 34 in a bit pattern generating circuit 11 of a reference signal generator 10, and reference data C corresponding to each pixel of the observation data A is prepared. Both the resultant signals are compared/collated with each other at each measurement position of the table 2. The above-described processing is performed while the table 2 is continuously moved at a constant speed.
In the conventional inspection apparatus, the reference data C prepared from the design data B forms a very accurate image as compared with the image formed by the observation data A. For this reason, when the pattern based on the reference data C is compared with the actual pattern, their difference is increased, especially at corner portions of the patterns, and a defect may be determined. More specifically, as shown in FIGS. 35a to 35c, the original graphic pattern (FIG. 35a) obtained by bit pattern development of design pattern data has a clear pattern edge and a clear corner shape. In contrast to this, both the pattern edge and corner shape exhibited by the observation data (FIG. 35b) picked up by the optical sensor 4 are blurred and rounded. If, therefore, they are simply compared with each other, as indicated by the comparison result (FIG. 35c), large errors are detected at portions corresponding to the corner and the edge, and a defect is determined.
In the actual manufacture of a mask, however, the corners of a pattern are usually rounded, and a certain degree of rounding has no influence on the electrical characteristics of a semiconductor integrated circuit. For this reason, it is preferable to proceed with a inspecting operation without determining such a rounded corner of a mask pattern as a defect.
In the apparatus shown in FIG. 33, in order to compensate for blurring based on the aperture characteristics of a lens, interference between adjacent pixels in a sensor, and the likes caused in an observation optical system, weighting/addition and multivalue conversion of the reference data C are performed by a point spread function in a distribution function calculator 13 to approximate the overall rounding (blurring) of the observation data A, thus obtaining reference data E. In addition, a feature extracting circuit 14 is used to extract a feature indicating whether a graphic pattern in an observation region white is being tested is a corner or a portion other than a corner, i.e., an entirely black pattern, an entirely while pattern, or an edge portion of a pattern, and an error threshold value F used for comparing/testing processing is updated that each feature so that a small difference in shape between patterns is not determined as a defect. This means that updating the threshold value F is effective in preventing the occurrence of false-defects especially at corner portions of a pattern.
The following problem, however, is posed in an apparatus of this type. In a mask pattern having a rounded corner, the threshold value range at the corner must be set to be broad in order to prevent the occurrence of a false-defect. If, however, the threshold value range is excessively widened, even a defect which is present near the corner and must be discriminated cannot be detected.
As described above, in the conventional pattern testing apparatus, since a corner of an actually manufactured mask pattern is generally rounded, even a portion which is not a defect (false-defect) is determined as a defect upon comparison between reference pattern data and pattern data to be inspected. In addition, if the threshold value range for comparison at the corner is excessively widened, a defect which is present near the corner cannot be detected.